Gamma-Ray Exposure Maps from Airborne Spectrometric Data, Kingdom of Saudi Arabia
Ian C.F. Stewart and Hani M. Zahran
Data from 16 airborne radiometric surveys over parts of the Saudi Arabian shield as well as most of the Phanerozoic cover rock survey within 200 km of the shield have been used to estimate the gamma-ray exposure risk arising from the surface geology. This does not include exposure from other sources such as radon. The shield survey data were obtained by digitizing hard-copy contour maps of potassium, thorium and uranium concentrations for 15 of the surveys, while only hard-copy total-count maps were available for the Makkah area. After editing the data and correcting the coordinates to the WGS84 spheroid, surface concentrations of the three elements were estimated from the survey calibration factors and used with standard equations to obtain grids of exposure values, which were plotted and compared with the mapped geology. Some additional point values for exposure were also derived from earlier ground data by calibrating the observations in counts per second against the airborne values. While the calculated risk values are generally quite low (less than 1 mSv yr-1) over most of the surveyed area, there are some very localized areas where the values exceed this, with a maximum from the airborne data of about 7 mSv yr-1 near Jabal Sayid. Due to geometrical effects in airborne radiometric surveying, even higher values could therefore be expected in some places from ground measurements. Most of the high values in the shield are associated with granitic lithologies. Over the cover rocks, the maxima tend to be immediately adjacent to the shield, probably due to alluvium and lower Palaeozoic lithologies derived from the shield rocks. Within the cover rock sediments relatively high risk values (up to 2 mSv yr-1) occur towards the base of the Qasim formation. Ground measurements in the area of high exposure anomalies in the airborne data to the south of Jabal Sayid show that values in excess of 150 mSv yr-1 occur due to thorite-rich pegmatites. The ground anomalies are more than 20 times the airborne values, and are also displaced by several hundred meters, probably due to location errors and geometrical factors. Although there may be appreciable errors in the older survey data, in both amplitude and location, the airborne data do provide a useful indication of anomalous areas worthy of further study. These results show that any high values in the airborne data should be checked by ground measurements. High exposures of up to 25 mSv yr-1 have also been found from direct measurements on drill cuttings from the Qurrayah prospect in the northwest of the shield, but this was not covered by any of the airborne surveys. The airborne data are far from complete, as only about 10 percent of the shield is included, and some significant localities are likely to have been missed, as shown by the Qurrayah measurements. Despite the limitations and possible errors in the airborne data, it has enabled preliminary risk estimates to be obtained over some of the more radiogenic areas of the shield as well as much of the cover rock region. These should serve to highlight some locations for further follow-up work including more ground studies.
Stewart, I.C.F. and Zahram, H.M., 2006, Gamma-ray exposure maps from airborne spectrometric data, Kingdom of Saudi Arabia: Saudi Geological Survey Open-File Report SGS-OF-2006-9, 60 p., 40 figs., 3 tables.